Thyristors are solid-state devices that provide flexible, reliable, and very fast control over voltages and currents. The main benefit of thyristors is that their conductivity may be controlled externally. Silicon Controlled Rectifiers (SCRs) are silicon-based thyristor units that are controlled by applying a triggering impulse to their gate terminal. SCRs act as open circuits until a triggering impulse is applied to the gate, at which time they begin to act similar to short circuits. Several SCRs (often six) may be arranged into a power bridge to provide controllable power conversion. The SCRs in a power bridge may be operated by sophisticated electronic systems that can be tailored to accommodate a great variety of electrical system hardware or power output requirements. Modern power generation systems rely heavily on SCR-based power bridges to provide a reliable and consistent supply of electricity to consumers.
SCRs can be cylindrically puck shaped, and SCR power bridges can be arranged in a modular stack system, which may be added to an existing power generation unit. Stack systems are typically more compact than other SCR power bridges. They also provide a safety benefit in that the stack may be encased in a housing that prevents accidental contact with the electrical devices, and may be provided with lockout equipment that helps ensure that the power is off when the cabinet is opened.
Ferrite cores can be placed in series with the main conducting paths of the SCR stacks to control the stresses placed on the SCRs. They effectively constitute a single turn inductor and may, among other things, help protect against an excessive rate of rise of the voltage (dv/dt) or an excessive rate of rise of the current (di/dt) that is applied to an SCR. While the SCRs can be clamped in the stack to heat sinks and insulators with a high mounting pressure, the ferrite cores are ordinarily mounted physically outside of the SCR stack. For example, FIG. 1 shows a configuration of a SCR stack diametric cell 102. The SCR stack diametric cell 102 is clamped between two bus bars 112 and 114 that may constitute two direct current outputs of a power conversion device. There may be two additional bus bars 116 and 118 that may constitute an alternating current input of a power conversion device. These two bus bars may be connected through an electrical connection 124 that comprises bends and increased mechanical complexity. An insulator 140 can be provided in the SCR stack 102 to provide an isolation mechanism for the bus bars, among other considerations. Attached to each of the bus bars, and external to the SCR stack, are the four ferrite cores 104, 106, 108, and 110 that may be used to protect the SCRs. In related art they comprise two U-shaped magnetically permeable structures, separated by NOMAX or another aramid material. In some applications, there may be no aramid material used between the two halves of the core, depending upon the required characteristics of the inductor. Also, in some applications, the ferrite material may be molded in one continuous piece with a hole in the center, instead of two halves. They may be placed around a bus bar, or other main SCR stack input/output means, that is made of copper or another electrically conductive material. The SCRs 120 may be separated by heat sinks 122 that dissipate heat into the ambient air. Stack-mounted SCRs have many important characteristics such as being easily removed, repaired, replaced, or otherwise serviced, and the external inductors allow for valuable protection of the SCRs, but they have their drawbacks. Among other limitations, externally mounted ferrite core SCR stacks may have a high production cost, low reliability, and may be mechanically difficult to assemble. Additional components, such as the insulator 140, bus bar 116 and electrical connection 124 are also needed.
The present invention overcomes the problems discussed above, and provides additional advantages, by employing a thyristor stack system with internally mounted reactors. It comprises two legs that are operatively connected between a first and a second bus bar, and the second bus bar and a third bus bar, respectively. Each of the legs further comprises a first thyristor operatively connected between a first and a second electrically conductive bar, each of the first the second electrically conductive bars being encircled by a first and a second magnetically permeable structure, respectively. The magnetically permeable structures may be ferrite toroids, and the thyristors may be silicon controlled rectifiers. Heat sinks may also be operatively connected between the thyristors and the electrically conductive bars. The system may also be employed in a load commutated inverter static starter power conversion unit.